Animal circular ssDNA viruses are a group of viruses of pathogenic importance. Human circular ssDNA viruses have been detected in patients with hepatitis of unknown aetiology. The titre of human TTV virus is also significantly higher in HIV-infected patients with AIDS, AIDS patients with a low CD4 T cell count, or patients with high HIV viral loads [Shibayama et al. (2001) AIDS 15, 563-570]. Touinssi et al. [J. Clin Virol. (2001) 21, 135-141] report a relationship between the prevalence of elevated viral loads of TTV virus and the level of immunocompetence of the populations studied and suggest that stimulation of the immune system by an interferon treatment was able to clear TTV viraemia. Moreno et al. (World J Gastroenterol (2004) 1, 143-146) however found that administration of PEG-IFN plus ribovarin could not induce a TTV sustained response in patients infected with hepatitis C.
Porcine circoviruses are associated with the occurrence of postweaning multisystemic wasting syndrome (PMWS) in pigs. Porcine circovirus 2 (PCV2) is a member of the family of Circoviridae. It is a very small virus with a relatively simple structure. The PCV2 viral genome does not code for a viral DNA-polymerase, making it dependent on cellular enzymes to complete its infectious cycle.
When PCV2 is inoculated in susceptible pigs, a high variation in virus replication is observed. It has been observed that PCV2 is able to replicate better in a host that is simultaneously inoculated with other viruses such as porcine reproductive and respiratory syndrome virus (PRRSV) [Allan et al. (2000) Arch Virol. 145, 2421-2429] or porcine parvovirus (PPV) [Allan et al. (2000) J Vet Med B. 47, 81-94]. A general stimulation of the immune system with keyhole limpet hemocyanin [Krakowka et al. (2001) Vet Pathol. 38, 31-42] has also been found to ameliorate the replication of PCV2 in the host.
Many cytokines are capable of modulating the susceptibility of the host to a viral infection. The most studied and best understood is the anti-viral effect of type I interferons (IFN-alpha and IFN-beta). But also other cytokines such as tumour necrosis factor alpha (TNF-alpha) and type II interferon (IFN-gamma) have been shown to influence infection. The antiviral effect of interferons has been demonstrated for many viruses and has been found to be so consistent and potent that humans and animals are routinely administered recombinant interferon (IFN-alpha) for the treatment of viral infections.